A gas-phase reactor often includes a reaction chamber, a susceptor to support one or more substrates within the reaction chamber, a gas distribution system, and an opening, such as gate valve, to allow loading and unloading of the substrates into or out of the reaction chamber and sealing of the reaction chamber during processing. During various gas-phase processes, the substrates can be heated to facilitate a reaction on a surface of the substrates—especially relative to the reaction on a surface of the reaction chamber—by heating the susceptor heater assembly. The substrates can be heated using a susceptor heater assembly that includes the susceptor and a heating device, which can be embedded in a portion of the susceptor. A reactor in which the substrates are heated, but in which the reaction chamber walls are not heated or are heated to a much lesser degree, is often referred to as a cold-wall reactor.
Cold-wall reactors can suffer from uneven heat distribution across a surface of a susceptor heater assembly and consequently across one or more substrates on the susceptor heater assembly. The uneven heat distribution can lead to temperature difference across a substrate during processing, which in turn, can lead to uneven film deposition, etch, clean, or the like processing on the substrate surfaces. At least part of the reason for the uneven heat distribution can arise from uneven heat flux from the susceptor heater assembly. Accordingly, improved apparatus and methods for providing heat across a surface of a susceptor heater assembly and across surfaces of one or more substrates on the susceptor heater assembly, while reducing uneven heat flux from the susceptor heater assembly and/or substrate, are desired.